1. 4 Copyright © 2004, Oracle. All rights reserved. Reporting Aggregated Data Using the Group Functions

2. 4-2 Copyright © 2004, Oracle. All rights reserved. Objectives After completing this lesson, you should be able to do the following: Identify the available group functions Describe the use of group functions Group data by using the GROUP BY clause Include or exclude grouped rows by using the HAVING clause

3. 4-3 Copyright © 2004, Oracle. All rights reserved. What Are Group Functions? Group functions operate on sets of rows to give one result per group. EMPLOYEES Maximum salary in EMPLOYEES table …

4. 4-4 Copyright © 2004, Oracle. All rights reserved. Types of Group Functions AVG COUNT MAX MIN STDDEV SUM VARIANCE Group functions

5. 4-5 Copyright © 2004, Oracle. All rights reserved. SELECT [column,] group_function(column),... FROM table [WHERE condition] [GROUP BY column] [ORDER BY column]; Group Functions: Syntax

6. 4-6 Copyright © 2004, Oracle. All rights reserved. SELECT AVG(salary), MAX(salary), MIN(salary), SUM(salary) FROM employees WHERE job_id LIKE '%REP%'; Using the AVG and SUM Functions You can use AVG and SUM for numeric data.

7. 4-7 Copyright © 2004, Oracle. All rights reserved. SELECT MIN(hire_date), MAX(hire_date) FROM employees; Using the MIN and MAX Functions You can use MIN and MAX for numeric, character, and date data types.

8. 4 Copyright © 2004, Oracle. All rights reserved. Example 1

9. 4-9 Copyright © 2004, Oracle. All rights reserved. COUNT(*) returns the number of rows in a table: COUNT(expr) returns the number of rows with non- null values for the expr : SELECT COUNT(commission_pct) FROM employees WHERE department_id = 80; SELECT COUNT(*) FROM employees WHERE department_id = 50; Using the COUNT Function 1 2

10. 4-10 Copyright © 2004, Oracle. All rights reserved. SELECT COUNT(DISTINCT department_id) FROM employees; Using the DISTINCT Keyword COUNT(DISTINCT expr) returns the number of distinct non-null values of the expr. To display the number of distinct department values in the EMPLOYEES table:

11. 4 Copyright © 2004, Oracle. All rights reserved. Example 2

12. 4-12 Copyright © 2004, Oracle. All rights reserved. Group functions ignore null values in the column: The NVL function forces group functions to include null values: SELECT AVG(commission_pct) FROM employees; SELECT AVG(NVL(commission_pct, 0)) FROM employees; Group Functions and Null Values 1 2

13. 4 Copyright © 2004, Oracle. All rights reserved. Example 3

14. 4-14 Copyright © 2004, Oracle. All rights reserved. Creating Groups of Data EMPLOYEES … 4400 9500 3500 6400 10033 Average salary in EMPLOYEES table for each department

15. 4-15 Copyright © 2004, Oracle. All rights reserved. You can divide rows in a table into smaller groups by using the GROUP BY clause. Creating Groups of Data: GROUP BY Clause Syntax SELECT column, group_function(column) FROM table [WHERE condition] [GROUP BY group_by_expression] [ORDER BY column];

16. 4-16 Copyright © 2004, Oracle. All rights reserved. SELECT department_id, AVG(salary) FROM employees GROUP BY department_id ; Using the GROUP BY Clause All columns in the SELECT list that are not in group functions must be in the GROUP BY clause.

17. 4-17 Copyright © 2004, Oracle. All rights reserved. Using the GROUP BY Clause The GROUP BY column does not have to be in the SELECT list. SELECT AVG(salary) FROM employees GROUP BY department_id ;

18. 4 Copyright © 2004, Oracle. All rights reserved. Example 4

19. 4-19 Copyright © 2004, Oracle. All rights reserved. Grouping by More Than One Column EMPLOYEES Add the salaries in the EMPLOYEES table for each job, grouped by department …

20. 4-20 Copyright © 2004, Oracle. All rights reserved. SELECT department_id dept_id, job_id, SUM(salary) FROM employees GROUP BY department_id, job_id ; Using the GROUP BY Clause on Multiple Columns

21. 4 Copyright © 2004, Oracle. All rights reserved. Example 5

22. 4-22 Copyright © 2004, Oracle. All rights reserved. Illegal Queries Using Group Functions Any column or expression in the SELECT list that is not an aggregate function must be in the GROUP BY clause: SELECT department_id, COUNT(last_name) FROM employees; SELECT department_id, COUNT(last_name) * ERROR at line 1: ORA-00937: not a single-group group function Column missing in the GROUP BY clause

23. 4-23 Copyright © 2004, Oracle. All rights reserved. Illegal Queries Using Group Functions You cannot use the WHERE clause to restrict groups. You use the HAVING clause to restrict groups. You cannot use group functions in the WHERE clause. SELECT department_id, AVG(salary) FROM employees WHERE AVG(salary) > 8000 GROUP BY department_id; WHERE AVG(salary) > 8000 * ERROR at line 3: ORA-00934: group function is not allowed here Cannot use the WHERE clause to restrict groups

24. 4-24 Copyright © 2004, Oracle. All rights reserved. Restricting Group Results EMPLOYEES … The maximum salary per department when it is greater than $10,000

25. 4-25 Copyright © 2004, Oracle. All rights reserved. SELECT column, group_function FROM table [WHERE condition] [GROUP BY group_by_expression] [HAVING group_condition] [ORDER BY column]; Restricting Group Results with the HAVING Clause When you use the HAVING clause, the Oracle server restricts groups as follows: 1.Rows are grouped. 2.The group function is applied. 3.Groups matching the HAVING clause are displayed.

26. 4-26 Copyright © 2004, Oracle. All rights reserved. SELECT department_id, MAX(salary) FROM employees GROUP BY department_id HAVING MAX(salary)>10000 ; Using the HAVING Clause

27. 4-27 Copyright © 2004, Oracle. All rights reserved. SELECT job_id, SUM(salary) PAYROLL FROM employees WHERE job_id NOT LIKE '%REP%' GROUP BY job_id HAVING SUM(salary) > 13000 ORDER BY SUM(salary); Using the HAVING Clause

28. 4 Copyright © 2004, Oracle. All rights reserved. Example 6

29. 4-29 Copyright © 2004, Oracle. All rights reserved. SELECT MAX(AVG(salary)) FROM employees GROUP BY department_id; Nesting Group Functions Display the maximum average salary:

30. 4 Copyright © 2004, Oracle. All rights reserved. Example 7

31. 4-31 Copyright © 2004, Oracle. All rights reserved. SELECT column, group_function FROM table [WHERE condition] [GROUP BY group_by_expression] [HAVING group_condition] [ORDER BY column]; Summary In this lesson, you should have learned how to: Use the group functions COUNT, MAX, MIN, and AVG Write queries that use the GROUP BY clause Write queries that use the HAVING clause

32. 4-32 Copyright © 2004, Oracle. All rights reserved. Practice 4: Overview This practice covers the following topics: Writing queries that use the group functions Grouping by rows to achieve more than one result Restricting groups by using the HAVING clause